qt: Only look for a framework on macOS if building for macOS
qt: Only look for a framework on macOS if building for macOS

When building for iOS, the Qt binaries only contain static libraries
and headers. No framework.

With this, Meson can successfully compile and link to Qt on iOS

QuLogic/meson,pexip/meson,mesonbuild/meson,becm/meson,jpakkane/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,becm/meson,jpakkane/meson,jpakkane/meson,QuLogic/meson,becm/meson,becm/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,becm/meson,becm/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,mesonbuild/meson,becm/meson,QuLogic/meson,jpakkane/meson,becm/meson,pexip/meson,jpakkane/meson,pexip/meson,pexip/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,becm/meson,mesonbuild/meson,pexip/meson,becm/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,jpakkane/meson